1. Field of the Invention
The present invention relates to an X-ray source which emits fluorescent X-rays of the type having an electron source and an anode target for the generation of X-ray bremsstrahlung, which produces mono-energetic X-radiation upon striking a fluorescent target.
2. Description of the Prior Art
An X-ray source is described in German OS 33 26 737 for the generation of fluorescent X-rays of different radiation spectra, in which an annular primary source, such as an americium radiator is provided in the cover of a housing, opposite a number cf fluorescent targets. A fluorescent target carrier plate carries the fluorescent targets, which are respectively composed of a variety of materials. Substantially mono-energetic fluorescent X-radiation is released by radiation from the primary source striking a selected, oppositely arranged fluorescent target. The fluorescent rays exit through a bore in the center of the primary source in the cover of the housing. Such a fluorescent X-ray emitting source is constructed very compactly, which accommodates common applications, but it cannot deliver a high flow of X-rays, since the activity of the primary source is typically at 10 mCi, nor can it be switched off, since the primary source constantly radiates. In addition, due to the continuously radiating (radioactive) primary source, it also must be cleared of radioactive waste, which is expensive. There are even considerable expenses in the transport of such fluorescent X-ray emitting sources, for reasons of safety.
There are also known deactivatable fluorescent X-ray emitting sources. Deactivability is achieved by using a deactivatable electron source, for example, a thermionic cathode supplied with a heating current that can be interrupted. Such fluorescent X-ray emitting sources are thus problematic, because he conversion of the electron energy into X-ray bremsstrahlung, and the subsequent conversion of X-ray bremsstrahlung into fluorescent X-radiation, each occur with only a low degree of efficiency. The first degree of efficiency equals approximately 1%; the second depends basically on the geometry of the arrangement. In order to achieve the image quality that is achieved with conventional X-ray tubes (the Detective Quantum Efficiency (DQE) is a significant parameter here), the electrical power would have to be increased to at least 10 times the electrical power common today, which is practically out of the question in practice.
In known fluorescent X-ray emitting sources of the abovementioned type, either the electrons which are generated by a cathode that is located in a focusing head are focused directly onto a fluorescent target, which naturally limits the electric power, or the electrons are accelerated onto one part of the housing of the X-ray source, where they generate bremsstrahlung, which then strikes the fluorescent target at a solid angle prescribed by the geometry of the arrangement, and generates fluorescent X-rays at this target (cf. European Application 0 459 567, German OS 37 16 618 and German OS 196 39 241). The previously mentioned problem of a low degree of efficiency arises in these devices as well, and so, to the extent that acceptable dimensions are maintained, only an unsatisfactory image quality can be achieved with fluorescent X-ray emitting sources of the abovementioned type. In the fluorescent X-ray emitting sources described in German OS 37 16 618 and German OS 196 39 24 cited above, a cone-shaped fluorescent target is used, which is irradiated by X-ray bremsstrahlung that is released by electrons from a focused electron source striking an anode target, so that the characteristic mono-energetic fluorescent X-radiation is released in the anode target.
Furthermore, in German OS 196 39 243 a fluorescent X-ray source with N different targets is described, which irradiates the different fluorescent targets with bremsstrahlung such that an electrical target changeover is possible. Aside from the fact that a number of focused electron sources is necessary for this, only 1/N of the maximum electrical power which is possible from the anode side is used.